1. Field of the Invention
The present invention relates to a linear actuator device.
2. The Prior Art
This kind of linear actuator device is used for beds and chairs with at least one adjustable section pivotally embedded in a frame. The section is raised and lowered by means of one or two rods mounted on a pivot axis. On the pivot axis an arm is in engagement with a spindle nut forming part of the actuator. The actuators are in the form of one embodiment in which a separate drive unit in a joint housing is placed at each end, or of another embodiment having only one drive unit. The first one of these embodiments is, e.g., known from EP 0 372 032 to Dewert and the second one from WO 2005/112709 A1 to Dewert.
Typically, the spindle nut is designed and shaped from plastic material as a quardrangular sliding element, with an arm on the pivot axis resting against one end face thereof. Normally, the arm is made from metal, and the spindle nut is highly exposed to wear at the position at which the arm rests against said spindle nut. The spindle nut will be made from plastic material following the requirements for low noise level as well as for minimum friction and also the demand for low manufacturing costs.
Also, there is the issue of the forces involved. As the arm pivots about a point thus describing a circular arc while, at the same time, the spindle nut performs a linear movement, this causes the place of engagement of the arm at the end of the spindle nut to move. During the introductory movement when the section is raised, the lever arm is short as the arm then lies high on the end of the spindle nut. During further movement the length of the moment arm increases and decreases at the end, when the section approaches its fully upright position. This resembles a moment curve, which rises from a certain level and then decreases again. Such a course of the moment curve is not particular expedient. The section also describes an arc of a circle, wherein the moment curve is largest when the section is in a horizontal position and the lever or moment arm decreases in correspondence with the raising of the section. Thus, at the beginning a large moment is needed, which does not match with the moment curve of the actuator. Towards the fully upright position a large moment is needed as well, which for a bed is due to the fact that the mattress which often is rather thick must be bent by a certain angle. This requires a moment of a considerable size. Ideally, the position of engagement of the arm would permanently lie exactly in the longitudinal axis of the spindle to avoid biassing of the spindle. For the largest possible torque applied on the respective section, the arm should be as long as possible. However, this is inconsistent with the requirement for minimum height of installation of the actuator.
In contrast to a fairly long arm at the end of the spindle nut, which, for example is known from EP 0 372 032 B1 to Dewert and DE 38 42 078 C2 to Okin, constructions have recently been suggested having a short arm with two legs riding astride the spindle and engaging both sides of the end face of the spindle nut, as, for example, is shown in WO 2005/110158 A1 to Okin. The problem concerning the length of the moment arm is, for instance, discussed in DE 296 07 493 U1 to Dewert. The document suggests a specific embodiment for both the arm and the spindle nut. The arm is constructed as an inverted V, with one leg being designed for engaging the front part of the spindle nut.
The other leg extends into an opening in the front part of the spindle nut and is intended for engaging a surface in the opening facing forward. During the pivoting movement one of the legs rests against the front surface of the spindle nut and is released at a given angle and the other leg engages the surface in the opening. This, to a certain degree, helps to balance the length of the moment arm.
The object of the invention is to achieve an enhanced course of the moment curve for the arm.